An imaging apparatus such as a printer, a facsimile machine, a copier, a plotter, or a printer/fax/copier multifunction machine may be a serial imaging apparatus as is described below or a line imaging apparatus including a line recording head, for example. A serial imaging apparatus includes a carriage in which a liquid discharge head configured to discharge droplets of recording liquid (e.g., ink) is arranged as a recording head (print head), and is configured to drive the carriage to serially scan a recording medium in a direction perpendicular to the conveying direction of the recording medium (also referred to as ‘paper’, ‘recording paper’, or ‘transfer material’ hereinafter). The serial imaging apparatus is configured to intermittently convey the recording medium according to a recording width, and form (record/print) an image on the recording medium by repeatedly alternating between conveying the recording medium and recording an image thereon.
An imaging apparatus as is described above that uses a liquid discharge head may realize multi-level tone printing by configuring the liquid discharge head to discharge liquid droplets in different sizes (e.g., small droplet, medium droplet, and large droplet) to thereby form dots in different sizes.
For example, Japanese Patent No. 3264422 discloses an imaging apparatus that uses a drive waveform including a first drive pulse for discharging a first ink droplet within a printing period and a second drive pulse for discharging a second ink droplet of a different size from the first ink droplet within the printing period, and combining the first pulse and the second pulse to selectively form dots in four or more different tones.
According to the above disclosure, print data of one bit per channel (1 bit/CH) are transmitted to a head driver for every drive pulse of a drive waveform. In this example, one channel refers to a unit including one nozzle, its corresponding liquid chamber and pressure generating means.
In the following, the data transmitting method used in the above disclosure is described in detail below with reference to FIG. 1. Print data are serially transmitted by a clock and registered in a shift register. The print data are latched by a latch signal (LAT) having a timing corresponding to the interval of the drive pulse so that the on/off state of an analog switch (switch means) for each channel may be determined. Specifically, previous print data are reflected in the current print data (e.g., if the current print data corresponds to medium droplet, print data transmitted for large droplet are reflected). In this example, a portion of a common drive waveform is applied to pressure generating means (piezoelectric element) to discharge liquid droplets of different sizes.
However, according to the above drive method, data transmission has to be completed at periods corresponding to the period of the drive pulse with the shortest pulse width used for on/off switching. Therefore, data transmission may be constrained by the requirement of a high clock frequency for data transmission, for example. Also, when the number of channels is increased in order to increase the printing speed, the pulse width of the drive pulse is decreased, and thereby, the data transmission time is rate limiting with respect to the printing speed. On the other hand, it is noted that according to this data transmitting method, since the switch means may be selectively switched on/off for every drive pulse, in principle, the number of tone levels may be increased to 2n (n representing the number of drive pulses within one printing period).
Japanese Patent No. 3219241 discloses a technique for decreasing the data transmission amount and the number of signal lines by providing storage means (shift register) and translating means, and controlling the operation of switching means based on print data decoded from a common drive waveform made up of plural drive pulses arranged within one printing period. By providing the translating means, data transmission of print data for one printing period may be realized at two bits per channel (2 bits/CH) in the case of four-level tone printing, for example.
According to the above-described data transmitting method, data are transmitted once in every printing period rather than in every drive pulse interval. Thereby, data transmission time appropriate for the number of channels and printing speed may be realized, and the load of the data transmission unit may be reduced. According to the present example, in the case of realizing data transmission at 2 bits/CH, the number of tone levels that may be realized is limited to four levels (i.e., large, medium, small, null). It is noted that the number of tone levels may be increased by increasing the number of bits (e.g., eight tone levels may be realized at 3 bits/CH); however, increasing the number of bits leads to an increase in the number of circuit elements within the head driver which results in cost increase as well as an increase in the data transmission amount.
Japanese Laid-Open Patent Publication No. 2003-1817 discloses a technique involving transmitting tone data and selecting a portion of a common drive waveform according to a corresponding control signal for the tone data.
In the following, the above-described data transmitting method is described in detail with reference to FIG. 2. Two bits of tone signals 0 and 1 are transmitted on separate signal lines and registered in a shift register. The tone (i.e., large, medium, small, or null) of each channel is determined by the first latch signal of a printing period. The tone of each channel determines the on/off state of a corresponding switch based on its corresponding control signal. Specifically, when the tone of a channel is set to large droplet (1, 1), an on/off state of an analog switch is determined based on a signal transmitted on a large droplet line of control signal lines.
It is noted that this transmitting method is similar to the method illustrated in FIG. 1 in that different waveforms are selected for large/medium/small droplets; however, in the present transmission as is illustrated in FIG. 2, the waveform selection switching may be realized even when the required processing time for a medium droplet is shorter than the data transmission time, and the data transmission amount may be reduced.
It is noted that the techniques disclosed in Japanese Patent No. 3219241 and Japanese Laid-Open Patent Publication No. 2003-1817 employ different methods for switching on/off an analog switch according to each tone; however, the two techniques both employ the data transmitting method involving transmitting tone data once in every printing period and selecting the portion of a common waveform to which pressure is to be applied based on the transmitted tone data.
It is noted that an imaging apparatus may be configured to realize non-interlaced one-pass printing as a high speed printing mode. In this printing mode, an image may be formed in one scan so that high speed printing may be realized.
In non-interlaced printing, the resolution in the sub scanning direction is arranged to equal the nozzle pitch of the recording head. Accordingly, in this case, the largest droplet size (e.g., large droplet) has to have a corresponding droplet discharge amount that is sufficient for forming a solid image.
However, in a recording head using an electromechanical conversion element (e.g., piezoelectric element), the machinability is limited to the nozzle pitch, and thereby, the resolution may not be adequately increased. With the present technology, the resolution is limited to approximately 180 dpi for one-line printing, and approximately 360 dpi for two-line-staggered printing, for example. In this case, a discharge amount of approximately 30-50 pl (the amount is subject to change depending on the type of paper/ink used) is required in order to form a solid line image in one scan.
On the other hand, with the demand for higher image quality, techniques are being developed for obtaining a smaller sized droplet in high image quality mode. It is noted that the discharge amount for a small droplet is preferably less than 2 pl at approximately 1.5 pl, and is expected to be reduced further to approximately 1 pl in future applications.
As can be appreciated from the above descriptions, an imaging apparatus that enables changing the droplet size (discharge amount) over a wider range is in demand.
However, in the case of varying the discharge amount over a wide range, it may be difficult to control the droplet size through the discharge of one droplet. Accordingly, in many cases, a large droplet is formed by discharging plural droplets of recording liquid. It is noted that with the increase in the range for varying the droplet sizes, there is a growing tendency towards decreasing a discharge amount Mj per one droplet and increasing the number of droplets to be discharged for realizing a designated droplet size. Also, it is noted that instead of simply discharging a number of droplets, each droplet may be merged before being adhered to paper in order to facilitate forming a solid image in the sub scanning direction.
When the number of droplets discharged within one printing period is increased, the discharging interval (pulse interval) is reduced. In such a case, it may be difficult to realize transmission of print data at every drive pulse as is described above (e.g., Japanese Patent No. 3264422), and data transmission involving transmitting tone data of two or more bits and employing translating means and selecting means (e.g., Japanese Patent No. 3219241, and Japanese Laid-Open Patent Publication No. 2003-1817) may be preferred.
However, even when the data transmitting method as is disclosed in Japanese Patent No. 3219241 and Japanese Laid-Open Patent Publication No. 2003-1817 is used, the following problems may arise. Specifically, when the number of tones is greater than or equal to the number of drive pulses (=number of discharged droplets) generated within one printing period, there may be a great difference in droplet size from one tone level to a next tone level.
For example, FIG. 3 shows a case in which tone data of four tone levels (two bits) are transmitted using a drive waveform having four drive pulses (referred to as first, second, third, and fourth pulses, respectively). In this case, there is at least a difference of two pulses (two droplets) between one tone level and a next tone level. In this example, the first pulse is selected for a small droplet, the first and second pulses are selected for a medium droplet, and the first through fourth pulses are selected for a large droplet; that is, there is a difference of two droplets between the medium droplet and the large droplet.
When a difference in droplet size between one tone level to a next tone level is large, inconveniences are created in realizing a smooth continuous-tone image, and the graininess of the large droplets may stand out from a halftoning process.
Such a problem becomes prominent as the droplet size of a small droplet is decreased and the number of droplets to be discharged for forming a large droplet is increased. For example, FIG. 4 shows a case in which tone data of four tone levels (two bits) are transmitted using a drive waveform having six pulses (referred to as first through sixth pulses, respectively). In this example, the first pulse is selected for a small droplet, the first through third pulses are selected for the medium droplet, and the first through sixth pulses are selected for the large droplet. Accordingly, there is a difference of two droplets between the small droplet and the medium droplet, and there is a difference of three droplets between the medium droplet and the large droplet.
In such a case, the number of tone levels may be increased to thereby control the droplet size in greater detail; however, increasing the number of tone levels, namely, increasing the number of bits leads to cost increase. In other words, although more detailed control of the droplet size may be realized by increasing the number of droplets to be discharged, in practice, such detailed control of the droplet size is restricted by limitations in the number of tone levels, for example.
Also, it is noted that an imaging apparatus may be arranged to form an image using differing resolutions for main scanning and sub scanning. For example, main scanning may be performed at 600 dpi, and sub scanning may be performed at 300 dpi. It is noted that in a serial imaging apparatus, when the nozzle alignment direction is in the sub scanning direction, although the resolution in the sub scanning direction is determined by the nozzle pitch, the resolution in the main scanning direction is determined by the carriage speed and the printing period. Therefore, the resolution in the main scanning direction may be adjusted with relative ease through adjusting the carriage speed. In this way, the resolution in the main scanning direction may be increased.
FIGS. 5A and 5B show examples in which images are formed at (main scanning resolution)×(sub scanning resolution)=300 dpi×300 dpi, and FIGS. 6A and 6B show examples in which images are formed at (main scanning resolution)×(sub scanning resolution)=600 dpi×300 dpi.
As can be appreciated from the illustrated examples, by increasing the resolution in the main scanning direction, the discharge amount for forming a large droplet may be reduced. However it is noted that even when the resolution in the main scanning direction is doubled as in the example of FIG. 6A, since ink has to be adequately applied in the sub scanning direction to form a solid image, the discharge amount for forming the large droplet may not be reduced to half the amount used in the example of FIG. 5A. Also, in order to maintain the printing speed in the example of FIGS. 6A and 6b to that of the example of FIGS. 5A and 5B, the drive frequency for the recording head has to be doubled so that difficulties may be created with respect to discharge control. For example, it is more difficult to control discharge of a liquid droplet of 20 pl at a drive frequency of 20 kHz compared to a case of controlling discharge of a liquid droplet of 40 pl at a drive frequency of 10 kHz.
FIG. 7 is a graph indicating the increase in the total discharge amount (droplet volume Mj) of liquid resulting from increasing the number of droplets discharged (drive pulse number), and the droplet volume Mjp of the liquid droplet discharged by each of the drive pulses. As is illustrated in this drawing, the total droplet volume Mj for forming a designated droplet may increase in an approximately linear manner in accordance with the increase in the number of drive pulses (individual droplet number); however, the droplet volume of the individual droplets (before being combined into a single droplet) discharged by the respective pulses for forming the designated droplet vary in a manner such that the droplet discharged by a later pulse is greater in volume than that discharged by an earlier pulse.
Such a phenomenon occurs owing to the fact that in the case of discharging plural droplets, the meniscus of liquid rises by the discharge of a droplet so that droplets discharged later may be larger in volume. For example, in FIG. 7, six droplets are combined to form one single droplet of approximately 37 pl. In this case, the total droplet amount (volume) of the first three droplets discharged by the first through third drive pulses, respectively, is approximately 15 pl, which is approximately 40% of the total droplet volume of the droplet to be formed.
In the case of varying the resolutions in the main scanning direction and the sub scanning direction as is described with reference to FIGS. 6A and 6B (e.g., 600 dpi×300 dpi), the droplets have to be configured to form a solid image in the sub scanning direction, and thereby, even when the resolution in the main scanning direction is doubled, the discharge amount may not be reduced to half the original amount and difficulties may arise with respect to control.
However it is noted that by increasing at least the resolution in the main scanning direction, the range of densities that may be represented only by small droplets may be widened so that a smooth continuous-tone image may be realized and the graininess of an image may be prevented from being degraded by a large droplet that may stand out in the tone image. Also, shagginess correction may be realized on characters and generation of jagged lines may be prevented.
It is noted that the droplet discharging interval has to be controlled more strictly as the number of droplets discharged within one printing period is increased for forming a large droplet that can fill up a solid image region. When the number of droplets to be discharged is increased, it becomes impossible to wait until the pressure within the liquid chamber of the recording head is adequately attenuated before discharging a next droplet. Thereby, the droplets have to be efficiently discharged in accordance with the pressure vibration of the liquid chamber and the pressure of the liquid chamber has to be controlled so that the pressure vibration does not go out of control. In this respect, the time and voltage of a drive waveform for forming a large droplet has to be adequately controlled.